Spec for MSC4230: Flag for animated images

Signed-off-by: Kévin Commaille <zecakeh@tedomum.fr>

data/event-schemas/examples/m.room.message$m.image.yaml

@@ -7,7 +7,8 @@
         "h": 398,
         "w": 394,
         "mimetype": "image/jpeg",
-        "size": 31037
+        "size": 31037,
+        "is_animated": false
     },
     "url": "mxc://example.org/JWEIFJgwEIhweiWJE",
     "msgtype": "m.image"

data/event-schemas/examples/m.sticker.yaml

@@ -9,7 +9,8 @@
         "mimetype": "image/png",
         "h": 200,
         "w": 140,
-        "size": 73602
+        "size": 73602,
+        "is_animated": true
       },
       "h": 200,
       "thumbnail_url": "mxc://matrix.org/sHhqkFCvSkFwtmvtETOtKnLP",

data/event-schemas/schema/core-event-schema/msgtype_infos/image_info.yaml

@@ -34,5 +34,18 @@ properties:
     allOf:
       - $ref: thumbnail_info.yaml
     description: Metadata about the image referred to in `thumbnail_url`.
+  is_animated:
+    description: |-
+      If this flag is `true`, the original image SHOULD be assumed to be
+      animated. If this flag is `false`, the original image SHOULD be assumed to
+      NOT be animated.
+
+      If a sending client is unable to determine whether an image is animated,
+      it SHOULD leave the flag unset.
+
+      Receiving clients MAY use this flag to optimize whether to download the
+      original image rather than a thumbnail if it is animated, but they SHOULD
+      NOT trust this flag.
+    type: boolean
 title: ImageInfo
 type: object